Solid-State NMR\nStudy of Hydrochars Produced from\nHydrothermal Carbonization of Poultry Litter

Poultry litter (PL) hydrochars obtained at different\ntemperatures\nand charring times were characterized by solid-state <sup>1</sup>H, <sup>13</sup>C and <sup>31</sup>P nuclear magnetic resonance (NMR) spectroscopy. <sup>13</sup>C NMR spectra obtained with cross polarization (CP) and magic-angle\nspinning evidenced the chemical and structural changes suffered by\nPL during its transformation into hydrochar; these changes were particularly\ndependent on the production temperature rather than the residence\ntime. The hydrochars were essentially composed of aromatic and alkyl\ndomains at the temperature of 250 °C. <sup>31</sup>P NMR observations\nwere conducted using single-pulse excitation (SPE) and CP sequences\nto distinguish between phosphorus far from protons and protonated\nphosphate species. Results showed that water-soluble phosphorus was\nthe only form detected in hydrochars through the CP sequence. In contrast,\nthe stable phosphorus species formed during hydrothermal carbonization\n(HTC) exhibited broad signals, detected exclusively using the SPE\nsequence. This indicates that unprotonated orthophosphates were the\ndominant form. These NMR results offer a deeper understanding of hydrochar\nformation from PL, shedding light on the chemical and structural changes\ncaused by the HTC process at the atomic scale.